(1) Field of the Invention
The present invention pertains in one aspect to an improved version of a dry thermal processor for extracting volatile substances from a particulate host material. The processor is of the type incorporating horizontal, concentric, substantially co-extensive, inner and outer tubular members which are interconnected and which rotate together about a horizontal axis. The feedstock enters at one end of the inner tubular member, advances through it, and is heated by hot solids returning through the annular space between the tubes.
In another aspect, the invention pertains to an improved version of the process wherein the feedstock is initially advanced through the inner tubular member and is heated in two stages, firstly to vaporize water contained in the feedstock and secondly to pyrolyse hydrocarbons and produce coked solids. The coked solids are transferred into the annular space, wherein the coke is burned to produce hot solids. Part of the hot solids is recycled into the hydrocarbon vaporization or reaction zone to provide needed heat for that zone. The balance of the hot solids is returned through the annular space and is used to transfer heat into the water vaporization or pre-heat zone by contact with the wall of the inner tubular member.
(2) Prior Art
The present invention relates to improved versions of the processor and the process disclosed in U.S. Pat. Nos. 4,280,879 and 4,285,773. This processor is known as the "ATP Processor".
The present application incorporates by reference the description of co-pending U.S. application Ser. No. 07/511,904, filed Apr. 23, 1990 by Taciuk et al.
The invention has to do with means for interconnecting the inner and outer tubular members. Those tubular members are subjected to different thermal environments, which create difficulties in their interconnection.
For completeness, a description of the difficulties and the efforts at solution made previously, leading to the present invention, are summarized herein.
The ATP Processor comprises inner and outer, generally tubular members herein referred to as tubes. The tubes are generally coextensive, concentric, spaced apart and horizontal. They are interconnected so as to form a unitary rotatable tube assembly. Stationary end frames seal the first and second ends of the outer tube. Drive means are provided for rotating the outer tube, and thus the entire tube assembly, about its longitudinal axis.
The arrangement and extraction process which takes place within the inner and outer tubes defines a plurality of environments which must each deal physically with particulate solids and variable temperatures. The general arrangement and purpose of these environments is described in the following.
A passageway extends longitudinally through the inner tube and an annular space is formed between the tubes. The inner tube passageway is closed at its first end by a stationary end frame and at the second end by a vertical closure plate. It is divided along its length by an upright baffle, thereby creating two segregated sequential chambers or "zones" which combine to extend between the first and second ends of the inner tube. The zone at the first end is referred to as the "preheat zone" and that at the second end as the "vaporization zone".
A feed stream comprising particulate solids may be fed into the first end of the preheat zone by means of a conveyor extending through the first end stationary end frame. As the tube assembly is rotated, this feed is advanced longitudinally through the inner tube passageway. As it is advanced, the feed is simultaneously cascaded and heated by heat exchange with the wall of the inner tube. The inner tube is heated by hot solids and flue gases moving countercurrently through the annular space. As a result of progressive heating of the feed during its advance through the preheat zone, the solids rise in temperature and contained water is vaporized. The produced steam is suctioned out of the preheat zone. The preheated feed is discharged from the preheat zone through helical chutes extending through the baffle. The chutes lead into the vaporization zone. On entering the vaporization zone, the preheated feed is mixed with hot solids recycled from the annular space. As a result, the feed is now heated to a relatively high temperature. The hydrocarbon associated with the solids is therefore vaporized and thermally cracked and some coke is formed on the solid particles. The hot gases are suctioned from the zone for recovery and treatment. The coked solids are discharged from the second end of the vaporization zone by means of a helical chute extending through the closure plate at the second end of the inner tube. The coked solids are discharged into the second end of the annular space.
The annular space provides combustion and cooling zones extending sequentially from the second end to the first end thereof. Air is injected through the second stationary end frame into the combustion zone. In addition, a gas burner also extends through the second end frame and supplies supplemental heat to the combustion zone. Lifters, extending inwardly from the inner surface of the outer tube along its length, lift and drop the coked solids through the injected air stream. In the course of this, the coke combusts and the solids are further heated. The majority of the heat is retained within by a refractory insulating layer, extending essentially along the length of the inside surface of the outer tube. The resulting hot solids are advanced longitudinally through the annular space from its second end toward its first end. A portion of these hot solids are recycled, by means of a chute, from the first end of the combustion zone into the first end of the vaporization zone, as was previously described. The balance of the hot solids advance into the annular cooling zone, which is coextensive with the preheat zone of the inner tube. Here the hot solids are repeatedly lifted and dropped onto the outer surface of the preheat section of the inner tube. Thus the preheat section is heated by contact with the shower of hot solids and the flow of hot flue gases moving through the cooling zone. At the same time the hot solids and gases are correspondingly cooled, thus recovering useful heat from them. The gases produced in the annular space are suctioned out and the cooled solids are discharged from the cooling zone through the first end frame by means of a chute.
The ATP process and apparatus is then characterized by a cool outer member, within which is supported a heavy, metal inner member which experiences significant thermal effects over its length. The contents of the inner member rise in temperature from about ambient temperatures of 70.degree. F. at its first end, to elevated temperatures of about 1000.degree. F. at its second end. The annular space formed between the members provides an environment which has a corresponding temperature profile of about 650.degree. to 1350.degree. F. More particularly, the metallic wall of the inner member thus rises in temperature from about 600.degree. to 1100.degree. F. The internally insulated, metal outer member generally operates at a relatively uniform and cool 200.degree. F., thereby minimizing heat losses and permitting drive components to be mounted thereto.
It is important to note that most materials, and particularly metals, possess a characteristic whereby the material expands and contracts as its temperature changes, generally expanding with temperature increase.
This then introduces a dilemma facing the designer of an inner and outer tube interconnecting means, wherein the differing temperatures therebetween result in severe differential thermal expansion effects. The inner tube thermally expands a greater amount than does the outer tube when operating in the hot mode, yet the outer tube must nonetheless be successfully and structurally connected together with some means extending through the annular space.
This differential thermal expansion is further aggravated by exposure of the actual interconnecting means to high temperatures. The interconnecting means itself expands a significant amount radially outwards, beyond the capability of the outer tube to respond.
If the differential thermal expansion is not compensated for, then yield stresses develop in the inner or outer tubes or the interconnecting means. If immediate failure does not occur, then subsequently, when these stresses are further superimposed on alternating stresses from the rotating action of the process, premature fatigue failure can result.
In early experimentation with a pilot-scale processor, the problem of differential thermal expansion was recognized but not successfully dealt with. Supports were attempted at three places; one main support at about the centre of the outer tube, and two others near each of the first and second ends of the inner tubes.
The first end of the inner tubular member was supported by spring washer-loaded support posts. These eventually failed and solid posts were welded in place. This approach was subject to eventual cracking of the weld sites. The support of the second end of the inner tubular member was originally a group of similar spring washer-loaded, inclined, multiple post supports. This latter assembly eventually failed as well and was replaced by multiple vertical post supports welded to the two tubular members.
The original configuration of a main, central connection of the inner and outer tubular members at the junction of the pre-heat and reaction zones was a leaf spring connected structure wherein differential radial motion flexed the springs in one plane, while inner member support was provided by the stiff section of the spring in the other plane. After significant operation, inspection of this area revealed cracked welds. Modifications were made to this area. More particularly, a plurality of internal pins, which were slidably receptive of radial growth, were installed but were restrained from axial and torsional movements by thrust blocks. This system lasted only a short time before the welds failed. Another modification was made. This second system involved a solidly welded structure offering some radial flexibility due to outer member solid blocks being welded in the middle of a wide flange which was radially offset and subsequently welded at either edge to the inner member. Post operation inspection has not yet revealed cracking at the connection sites, albeit at a low number of fatigue cycles for commercial acceptance.
Investigation of alternate design aspects for this main support area resulted in the conception of several solutions involving uncoupling the inner and outer tubular members and enabling free and independent movement of the tubular members in a radial direction with respect to each other, while preventing movement in the axial and rotational directions. These concepts produced mechanically complex arrangements, with link and pivot components prone to wear and a requirement for periodic replacement.
Recognizing the inherent simplicity and mechanical security of the rigid connection, it was determined that the key was not to accept differential radial expansion and work around it but to work with it and manipulate the intensity of differential movement.